This invention relates to a method of controlling fluid pressure on a load that is effective when e.g. automotive brake fluid pressure is controlled using a proportional pressure control valve or a flow-control valve.
From unexamined Japanese patent publications 1-261581 and 3-69877 and unexamined Japanese utility model publication 1-261581, as well as Japanese patent applications 9-133450 and 9-21611 filed by the applicant of this invention, proportional pressure control valves are known which have a drive means capable of electrically controlling the output and adapted to control the fluid pressure on the load to a value corresponding to the driving force of the drive means by moving a spool by the drive means to a position corresponding to the driving force of the drive means.
This type of proportional pressure control valves includes a housing having a first port connected to a fluid pressure source, a second port connected to a load, and a third port connected to a reservoir, a spool mounted in the housing, a first valve portion for opening and cutting off communication between the first and second ports depending on the position of the spool, and a second valve portion for opening and cutting off communication between the second and third ports.
Specifically, when the spool is moved to a point where the driving force of the drive means balances with a thrust produced by receiving in opposite directions the pressure at the second port on pressure receiving portions having different areas, the state of communication of the second port with the first or third port is changed over, and the degree of opening of the first or second valve portion is adjusted so that the pressure at the second port is controlled to a value corresponding to the driving force of the drive means.
FIG. 1 shows fluid pressure-fluid amount characteristics. As shown, automotive brakes consume a greater amount of brake fluid in a low-pressure region.
Thus, if a proportional pressure control valve of the above-described type is used to control automotive brake fluid pressure, control responsiveness worsens in the low-pressure region.
As shown in FIG. 2, in a region where a lesser amount of brake fluid is consumed (region where the pressure is higher than P1), pressure rise or fall per unit fluid amount is great, that is, the pressure rises and falls sharply. The actual pressure P thus approaches the target pressure P(n) quickly. But in a region where a large amount of brake fluid is consumed (region where the pressure is lower than P1), pressure rise or fall per unit fluid amount is small, that is, the pressure rises or falls only slowly. It thus takes a long time for the actual pressure P to reach the target pressure P(n). Responsiveness of control is thus no good.
Automotive brake systems have been sophisticated year after year. Some recent brake systems include not only an ABS (antilock brake control system) but a TCS (traction control system) and an ASC (active safety control) system. Such systems require high-precision control even in the low-pressure region.
An object of this invention is to improve responsiveness of control in such a low-pressure region.